1. Field of the Invention
The invention relates to air-operated spring brake actuators and, more particularly, to a spring brake actuator with a sealed spring chamber and an internal vent mechanism for relieving pressure build-up in the spring chamber during release of the brakes.
2. Related Art
Air-operated brake actuators are commonly used in heavy-duty vehicles, such as semi-tractor/trailer combinations, to urge the brake pads against the brake drum and thereby apply the brakes of the vehicle. In most heavy-duty applications in North America, an S-cam brake assembly is the preferred brake system.
Air-operated brake actuators are either the piston type or diaphragm type. Typically, air-operated diaphragm brake actuators are arranged in a tandem configuration comprising an air-operated service brake actuator for applying the normal operating brakes of the vehicle, and a spring brake actuator for applying the parking or emergency brakes of the vehicle. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm dividing the interior of the housing into two distinct fluid chambers. A piston brake actuator operates under the same principles, except that instead of a diaphragm, a piston reciprocates in a cylinder.
In the case of the service brake actuator, the service brake housing is divided into a pressure chamber and a pushrod chamber. The pressure chamber is fluidly connected to a source of pressurized air and the pushrod chamber mounts a pushrod, which is coupled to the brake assembly, whereby the introduction and exhaustion of pressurized air into the pressurized chamber reciprocates the pushrod into and out of the housing to apply and release the operating brakes.
In the case of a spring brake actuator, the housing is divided into a pressure chamber and a spring chamber. A pressure plate is positioned in the spring chamber between the diaphragm and a large force compression spring, whose opposing end abuts the housing. In one well-known configuration, an actuator rod extends through the pressure plate, through the diaphragm, into the pressure chamber, and through a dividing wall separating the spring brake actuator from the service brake actuator. The end of the actuator is fluidly connected to the pressure chamber of the service brake actuator.
Under parked conditions, where the spring brake actuator pressure is fluidly connected to atmosphere, the large force compression spring will force the diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, the parking or emergency brakes are applied, and the vehicle cannot be moved. To release the parking brake, the pressure chamber is closed to the atmosphere and pressurized air is introduced into the pressure chamber of the spring brake actuator which expands the pressure chamber, moving the diaphragm and pressure plate toward the opposing end of the spring brake actuator housing, thereby compressing the large force compression spring.
One problem with spring brake actuators of this design is that as the large force compression spring is compressed, the pressure chamber increases in volume and the spring chamber decreases in volume, resulting in a pressure increase in the spring chamber, unless a system is provided for relieving the pressure increase. The build-up of pressure in the spring chamber upon the release of the brake is highly undesirable in that any pressure build-up in the spring chamber must be offset by an increased pressure in the pressure chamber if the spring is to be fully compressed to fully release the brake.
The pressure build-up in the spring chamber is exacerbated in that most pressurized air systems for heavy-duty vehicles operate at an industry standard maximum pressure. The combined pressure of the spring and the increase in air pressure in the spring chamber cannot approach the maximum for the brake to operate properly. As the combined force associated with the pressure of the spring and the build-up of pressure in the spring chamber approach the force applied by the maximum pressure, the brake can fail to release, only partially release, or release very slowly, all of which are undesirable.
One solution to the pressure build-up in the spring chamber is to vent the spring chamber. The most common venting mechanism since the invention of the diaphragm brake actuator is to place holes in the housing around the spring chamber. A great disadvantage of such vent openings is that the interior of the spring chamber is thus exposed to the external environment. Environmental elements such as dirt, salt, and water can then enter the spring chamber and accelerate abrasion, corrosion, or wear on the various internal brake components, especially the spring. The damage to the internal brake components by the environmental elements can cause increased maintenance or premature failure of the spring and consequent replacement of the brake actuator.
An additional problem with directly externally venting the spring chamber is that a tractor/trailer is often parked for extended periods in a bay adjacent the dock. The bays are typically sloped and below grade. Under heavy rain or snow conditions, a bay can fill with water to a height above the vent opening and flood the interior of the spring chamber. Although the water would normally be expelled from the spring chamber through the vent openings as the brake is released, the flooding can accelerate corrosion and introduce other environmental hazards. In certain environmental conditions, the water can freeze, which may prevent release of the brake altogether.
Because of the problems associated with the introduction of environmental elements into the spring chamber through the vent openings, attempts have been made to seal the spring chamber to prevent the introduction of the various environmental elements. Sealing the spring chamber, however, creates additional problems in that a vacuum or a lower pressure tends to form in the spring chamber when the parking brakes are applied, unless a system is provided for relieving the low pressure. If the low pressure is great enough, it can slow the response time of the parking brakes, which is not desirable.
Prior solutions to eliminating the pressure build-up and vacuum creation in the spring chamber while keeping out environmental elements include fluidly connecting the spring chamber of the spring brake actuator to either chamber of the service brake actuator, placing a filter in the vent opening, and providing an internal fluid flow path from the spring chamber through the actuator rod and into the service brake pressure chamber. All of these solutions are compromises in that they do not provide complete solutions or introduce other complicating problems. For example, the filtered vent openings inherently permit external air to enter the brake, yielding a brake than is not completely sealed. As long as the filter is open there is some possibility that external elements can enter the brake through the filter such as if the brake actuator is submerged in a flooded bay. An example of a filtered vent opening is found in U.S. Pat. No. 6,029,447 issued Feb. 29, 2000. The internal fluid paths extending through the actuator require complex two-way valves that control the fluid flow to release a pressure build-up in the spring chamber while permitting the introduction of pressurized fluid to prevent a vacuum in the spring chamber. Examples of such two-way valves are disclosed in U.S. Pat. No. 5,722,311, issued Mar. 3, 1998 and U.S. Pat. No. 5,372,059, issued Dec. 13, 1994.
It is desirable to have an air-operated brake actuator including a spring brake actuator wherein the spring brake actuator is sealed and the pressure increase and vacuum formation are remedied without the need for complex or high maintenance valve and filter systems.
An air-operated brake actuator according to the invention comprises a sealed housing having a first end wall, a peripheral side wall, and a second end wall opposing the first end wall. The housing defined by its walls enclose a spring brake cavity. A movable member spans the spring brake cavity and divides it into a spring chamber located between the movable member and the first end wall and a pressure chamber located between the movable member and the second end wall. The pressure chamber is adapted for connection to a source of pressurized fluid so that the movable member is in a first position when the pressure chamber is pressurized and a second position when the pressure chamber is exhausted. A spring is disposed in the spring chamber and biases the movable member toward the second end wall away from the first position. In the first position, the spring is compressed, and in the second position, the spring is less compressed.
A hollow actuator rod has one end coupled to the movable member and another end extending through the second end wall to establish fluid communication through the actuator rod between the spring chamber and a side of the second end wall opposite the pressure chamber. Thus, when the movable member is in the first position (spring compressed), the hollow actuator rod is adapted to release the parking brake, and when the movable member is in the second position (spring less compressed), the hollow actuator rod is adapted to apply the parking brake. A one-way valve is positioned within the hollow actuator rod to permit the exhaust of fluid from the spring chamber through the hollow actuator rod and prevent the introduction of fluid through the hollow actuator rod into the spring chamber. Thus, when pressurized fluid is introduced into the pressure chamber, the movable member moves from the second position to the first position, thereby reducing the volume in the spring chamber. The one-way valve opens in response to pressure build-up in the spring chamber above a predetermined pressure to relieve the pressure build-up by permitting the pressurized fluid to pass from the spring chamber through the hollow actuator rod.
In one aspect of the invention, the spring is sized to apply a spring force sufficient to negate the impact of a retarding force attributable to the spring chamber in an application time defined by the time for the spring to move the movable member between the first and second positions upon exhaustion of air from the pressure chamber. Preferably, the retarding force equals the force opposing the expansion of the spring attributable to the pressure differential between the pressure chamber and the spring chamber when the movable member is in the second position. Typically, the retarding force will be at least 175 lbs.
In another aspect of the invention, the one-way valve comprises a body having a through opening and positioned within the hollow actuator rod to block fluid flow through the hollow actuator rod except through the through opening. A poppet is mounted within the through opening and movable between a sealing position where the poppet seals the through openings to prevent the flow of fluid through the body, and an open position where the poppet unseals the through opening to permit the flow of fluid through the body.
Preferably, the body comprises a manual collar extending into the body through the through opening, and the one-way valve further comprises a biasing device biasing the poppet into abutting relationship with the collar to seal the through opening to place the poppet in a sealing position. The biasing device applies a force to the poppet that can be overcome by a cracking pressure of less than 2 psig across the poppet. Preferably, the cracking pressure is in a range between 0.5 and 1 psig. Typically, the one-way valve will have a retainer mounted to the body and a spring disposed between the retainer and the poppet.